The invention relates to electrically programmable read-only memory cells, what are known as flash EEPROM cells, which are not only erased but also programmed by Fowler Nordheim tunnels. For reasons of selectability, in these EEPROM cells the inception voltage is high, e.g. 5V, in the erased case, and low, e.g. 1V, in the programmed case. A high threshold voltage means that there is a high negative charge on the floating gate of the respective cell transistor. Conversely, given a low threshold voltage, the floating gate is not charged, or barely so. The control gate of a respective cell transistor is connected to a word line, and the drain terminal of a respective cell transistor is connected to a bit line. A programming occurs by the application of -10V at the word line and 5V at the bit line, for example. Sometimes, individual cells can be programmed particularly easily, which can lead to what is known as "overprogramming", whereby these cells comprise a negative threshold voltage consequent to a positive charge on the floating gate. In the readout of cells, the selected word line lies at 2.5V, the corresponding bit line lies at 1V, and the non-selected word and bit lines remain at 0V, for example. If the threshold voltage of the selected cell is low, then a bit line current flows under these voltage conditions, and the cell is recognized as programmed. If the cell has been erased, then a bit line current usually does not flow; however, if the threshold voltage is negative due to overprogramming, then a current does flow through these cells, even though they were not selected. If, for example, a cell is read out which is actually an erased cell, a false evaluation can result. For this reason, negative values of the threshold voltage must generally be avoided. In addition, for smaller operating voltages or multi-level applications, the distribution of the inception voltages of a cell field should be optimally low, and the value of the threshold voltage should be adjustable.
In IEEE Electronic Device Letters; Vol. 16, No. 3, March 1995: 121-123, a mechanism is taught in which the bit line can either be charged with 5V via a charge transistor or can be discharged to 0V via a discharge transistor. The problem of what is known as overprogramming is prevented by what is known as a self-converging programming. The latter occurs in that, during a negative phase of a pulse at the word line, the negative charge on the floating gate drops, and with it the inception voltage of the cell, and in a positive phase of the pulse of the word line, it is checked whether or not the desired threshold voltage has been attained yet. If the desired threshold voltage has been attained, the cell opens, and the bit line thereby discharges, whereby subsequent negative pulses do not effect further programming. However, measurements have demonstrated that the bit line is prematurely discharged due to leakage currents into the substrate, and thus an incomplete programming often occurs.
The IEEE Paper to the IEDM 96, 7.4.1 to 7.4.4, pages 181 to 184 teaches that the bit line is recharged by a weakly opened transistor, it being possible to avoid an overly low programming in this way. However, it is disadvantageous that this functions only given a very specific bit line voltage at the recharge transistor, and for even slight deviations of 0.1V, for example, either an overly low programming or a non-convergence at the desired threshold voltage value occurs. Another disadvantage is that the programming phase may not be longer than approximately 1 .mu.s, since otherwise the bit line is charged again via the recharge transistor during the programming of the bit line, and the cell is thus further programmed. Due to an overly low programming pulse width, many program/read cycles are necessary, which prolongs the total programming time unnecessarily.